1. Field of the Invention
The present invention relates to phase change memory and more specifically to the programming of a phase change memory device.
2. Description of Background
There are two major groups of computer memory: volatile memory and non-volatile memory. In volatile memory, constant energy input is required to retain information, while in non-volatile memory constant energy input is not required. Examples of volatile memory devices include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM). DRAM requires the memory element to be constantly refreshed (requiring energy) while SRAM requires a constant supply of energy to maintain the state of the memory element. Examples of non-volatile memory devices are Read Only Memory (ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric Random Access Memory, Magnetic Random Access Memory (MRAM), and Phase Change Memory (PCM).
As stated, the information in the memory elements of non-volatile memory can be retained for days to decades without power consumption. The present invention is directed to phase change memory; a type of non-volatile memory. In phase change memory information is stored in materials that can be manipulated into different phases. Each of these phases exhibit distinct electrical properties that can be used for storing information. An amorphous and a crystalline phase are typically two phases used for bit storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. This group of materials contain a chalcogen (Periodic Table Group 16/VIA) and another element. Selenium (Se) and tellurium (Te) are the two most common elements in the group used to produce a chalcogenide semiconductor when creating a phase change memory cell. An example of this would be Ge2Sb2Te5 (GST), SbTe, and In2Se3.
Altering the phase change material's state requires heating the material to a melting point and then cooling the material to one of the possible states. A current passing through the phase change material induces ohmic heating and causes the phase change material to melt. Melting and gradually cooling down the phase change material allows time for the phase change material to form the crystalline state. Melting and abruptly cooling the phase change material quenches the phase change material into the amorphous state.
In multi-bit storage, an individual phase change memory cell must be able to be programmed to multiple states. These multiple states are various ratios of amorphous phased and crystalline phased phase change material. The ratio of amorphous to crystalline phase change material directly affects the electrical resistance of the memory cell.
A problem in phase change memory is accurately programming a phase change memory cell to multiple states. The current necessary to program a memory cell to a specific resistance is dependent on its existing resistant. This poses a problem in accurately programming a phase change memory cell to a target resistance when the current required must be altered according to its existing resistance and target resistance. Thus, it is desirable to devise a method for programming a phase change memory device that can utilize consistent current levels for programming a phase change memory cell to a plurality of target resistances.